1. Field of the Invention
This invention relates to turbidity meters for the measurement of solids suspended or colour bodies dissolved in a fluid.
2. Review of the Art
Whilst the term turbidity strictly speaking refers to the light transmission and scattering properties of a liquid containing suspended solids, similar techniques to those used for measuring turbidity may commonly be used for determining the degree of opacity of coloured liquids and of gases containing suspended solid or liquid particles. For convenience, the term turbidity will be utilized hereinafter in a broad sense to refer collectively to all of these phenomena unless otherwise indicated.
Whilst direct measurements of turbidity of a fluid can be made by separating the turbidifying phase from the fluid and measuring the weight of both phases separately, this is time consuming and difficult. For most purposes, measurement of the optical properties of the turbid fluid provides a more convenient although indirect way of quantifying turbidity. Commonly measured properties for this purpose are the attenuation and scattering of light passed through the fluid. Measurements of scattering (nephelometry) are usually most suitable when the degree of turbidity and thus attenuation is low, whilst measurements of attenuation (turbidimetry) are most useful when the degree of turbidity and thus attenuation is high. The measurement range of most existing instruments is limited, so that no single instrument can provide a capability extending over the full range of turbidities likely to be encountered in the field.
A further problem arises in instruments having measuring heads which must be submerged in the fluid under test, since material from the fluid or biological growth engendered by the fluid will tend to deposit on windows or lenses associated with light sources and detectors incorporated in the head, thus introducing measurement errors. In instruments that make measurements within the fluid, rather than withdrawing samples for analysis, further errors arise through spurious responses of the light detectors as a result of ambient light.
Instruments currently available are calibrated in turbidity units (Jackson T.V. or N.T.V.) which are based on a standard suspension with reproducible optical properties which are not simply related to the content of suspended solids. The presence of colour due to dissolved solids is interpreted by most current instruments as turbidity. Such instruments are adjusted manually, and measurements are also taken manually or from chart records. Manual adjustments and readings at a measurement location are inconvenient at best and possibly hazardous or impossible: remote operation would be desirable.